Newton in the Pool Hall: Subtleties of the Third Law

In order to apply the Third Law of Motion to widely separated celestial bodies, Newton must confront the experimental behaviors of colliding billiard balls, despite the fact that such objects otherwise play little role within the Principia’s formal developments. This challenge forces Newton to confront the fraught question of the sources of their elastic rebound and to postulate implausible “hard atoms” as a result. Leibniz’s surprising opinions on this same topic are presented by way of contrast.